Pattern transfer is used for the fabrication of integrated circuits, microsystems, integrated sensors, and micromachined devices. Pattern transfer is comprised of two processes: lithography and etching. In lithography, the pattern to be transferred to a substrate is first transferred onto a polymer film referred to as photoresist film. The film is exposed to some form of radiation, and usually heat-treated after the exposure. Chemical changes in the polymer film are such that exposed areas can be differentiated from dark areas by treatment (wet development) in an appropriate solution. Development either removes the exposed regions (positive-tone process) or the unexposed regions (negative-tone process). This reveals the underlying substrate in some areas, while masking it in other areas.
The etching process is usually done in a plasma discharge, which etches away the substrate in the areas not masked by the remaining photoresist film. The etching must be anisotropic, ie., directed preferentially in one direction perpendicular to the surface of the substrate, in order to maintain the fidelity and the line width of the transferred pattern. Details of the patterning process can be found in many books, such as W. M. Moreau Semiconductor Lithography, Principles, Practices and Materials, Plenum Press 1992, which is incorporated herein in its entirety by this reference.
Photoresist films must have appropriate absorbance (typically less than 0.5) in order to be imaged to their entire depth. However, lithography is usually done on non-flat substrates with topography, where the resist thickness varies from one place on the substrate to another. As a result, line-width variations and limitations in the resolution of lithography occur, due for example to reflective notching. In other cases, the photoresist film is not transparent to radiation, as is the case, for example, with aromatic polymer-based photoresists and UV radiation below 200 nm. It is thus often advantageous both for highly and for non-absorbing photoresists to image with the radiation only the top part of the photoresist film (surface imaging).
Photoresist films must also have good plasma resistance, in order to protect the underlying substrate in the masked areas during the plasma-etching step. Since the etch resistance of organic, and especially aliphatic photoresists, is small, it is often advantageous to use organometallic photoresists (e.g. silicon containing), or to incorporate silicon in situ selectively in the photoresist film (silylation reaction), rendering the material resistant to oxygen or chlorine plasma.
To accommodate the above described advantages and/or the needs for surface imaging, and plasma resistance of photoresists, two approaches are often used:
1. A bilayer approach. Here the substrate is first covered with a thick planarizing organic polymer. On top of this polymer, a thin photoresist film is coated, exposed and wet developed. If the photoresist is an organometallic material (see for example M. Hatzalis, J. Paraszczak, J. Shaw, Proc. Microcircuit Engnrg. Lausane, page 396, 1981 for organosilicon materials), the structure can be dry-developed in an oxygen plasma. the areas of the planarizing polymer that are masked by the organosilicon photoresist are protected, while the others are etched away. Thus, the image is first transferred (etched) on the polymer layer, before the etching of the substrate is performed. If the photoresist does not contain silicon or other element that can form an in situ mask in an oxygen plasma, a silylation or other element compound treatment follows the wet development of the top photoresist. (see for example J. M. Shaw, M. Hatzakis, E. D. Babich, J. R. Paraszczak, D. F. Witman, K. J. Stewart, J. Vac. Sci. Technol. B 7 (6), 1709 (1989), U.S. Pat. Nos. 5,384,220 to Sezi et al, and 4,931,351 to McColgin et al., which are incorporated herein by this reference). 2) A single layer surface silylation process: Here after the exposure of the photoresist film, and usually following a heat treatment, differentiation exists between the exposed and the dark areas concerning their ability to react with the silylating agent. Exposed or unexposed regions can become crosslinked posing a diffusion barrier to the silylating agent (see for example U.S. Pat. No 5,312,717 to Sachdev, which is incorporated herein by this reference). Alternatively reactive functionalities are created or destroyed in the photoresist upon exposure (see for example U.S. Pat. No. 4,810,601 to Allen, which is incorporated herein by this reference). The photoresist is then treated with the silylating medium, which selectively silylates the exposed or the dark areas.
In both cases, the dry development of the bottom planarizing photoresist or non silylated part the silylated photoresist are done in an oxygen-atom containing plasma, in a reactive ion etcher or a high density plasma etcher apparatus with or without magnetic confinement. In addition to oxygen atoms, other atoms or gases can be present in the plasma to give the required anisotropy during the plasma-etching step. For example sulfur dioxide can be used for anisotropy (M. Pons, J. Pelletier, 0. Joubert, J. Appl. Phys. 75 (9), 4709 (1994), which is incorporated herein by this reference), or hydrogen liberating gases for avoidance of filament formation at the line edges of the patterns (see U.S. Pat. No. 5,041,362 to Douglas, which is incorporated herein by this reference). Moreover, the dry development is usually done in two steps. First, a short treatment with a plasma containing oxygen- and fluorine-liberating gases is performed, in order to remove the unwanted small quantities of silicon that have been incorporated in the crosslinked or non reacting areas and improve the silylation selectivity between exposed and dark areas. Secondly, a longer treatment with the oxygen plasma is performed (See E. Gogolides, D. Tzevelekis, S. Grigoropoulos, E. Tegou, and M. Hatzakis, J.Vac. Sci. Technol., B 14 (5), 3332 (1996), which is incorporated herein by this reference). Important is also the temperature of the photoresist during etching be kept low, since at temperatures greater than the glass transition temperature (T.sub.g) of the silylated resist, resist flow and pattern deformation can occur. Low temperature is also important for good anisotropy (M. Pons, J. Pelletier, O. Joubert, P. Paniez, Jpn. J. Appl. Phys. 34 Pt 1 No 7A, 3723 (1995), which is incorporated herein by this reference). Some workers also use cryogenic temperatures in order to ensure very high anisotropy.